Summary

The 2019 Southern Hemisphere stratospheric polar vortex weakening and its impacts

Scientific journal article

Scientists have summarised the record warming of the southern hemisphere stratosphere (SHS) that occurred in the spring of 2019. During several weeks from late August to mid-September 2019, the stratospheric westerly winds circling Antarctica (polar vortex) weakened and temperature in the stratosphere rose dramatically. This included record high mid-stratospheric

temperatures.

These conditions descended towards the earth’s surface from mid-October to December leading to a record-breaking negative phase of the Southern Annular Mode (SAM), a large-scale climate driver. The SAM is the latitudinal (north-south) movement of powerful westerly winds circling the southern hemisphere between 40° and 60° latitude. A negative SAM means the westerly winds are occurring at more northerly latitudes.

The weak polar vortex and associated negative SAM were the primary cause of extreme hot and dry conditions over subtropical eastern Australia in late spring to early summer 2019. These conditions led to the severe bushfires that occurred in late spring 2019. Both the weakening of the polar vortex and the resulting negative SAM were accurately predicted around late July with several state-of-the-art seasonal forecasting models. This was well communicated to governments, emergency services and affected industries. It is likely the losses associated with the 2019 fire season would have been considerably higher without this advance warning.